1. Field of the Invention:
The present invention relates to the isolation and purification of isocyanate condensates containing free NCO groups, prepared from aromatic di- or polyisocyanates, and wherein at least one of the free NCO groups is directly bonded to an aromatic ring.
2. Description of the Prior Art:
Isocyanate condensates are used for preparing foams, elastomers, adhesives, paints and varnishes, the exceptional properties of which condensates now being well known to this art.
These condensates, which contain free NCO groups, are prepared by reacting at least one compound containing at least two functional groups reactive with isocyanates, with a molar excess of a di- or polyisocyanate, optionally in a solvent which is inert with respect to NCO groups. The compound containing functional groups which are reactive with NCO groups may contain --OH, --NH.sub.2, --SH, --COOH, --NH, --CONH.sub.2 and --CO--NH-- groups as the reactive sites, with the proviso, of course, that such functional groups in said compound may be identical or different. Exemplary such compounds include the diols, polyols, aminoalcohols and di- and polyamines. The compound bearing the functional groups which are reactive with NCO groups may comprise an aliphatic, cyclic, cycloaliphatic or aromatic diradical; it may itself also be produced by the condensation of simple molecules and may result in a diradical comprising hetero atoms in the chain, if appropriate. Such condensates may be:
(i) polyesters produced by esterification of one or more di- or polyols with one or more di- or polyacids, or by the reaction of a cyclic lactone with a di- or polyfunctional molecule containing --OH, --NH.sub.2 or --NHR groups, for example:
(ii) polyethers produced by condensation of cyclic oxides (ethylene, propylene, butylene or tetramethylene oxide) with a di-- or polyfunctional molecule containing -OH, --NH.sub.2 or NH groups;
(iii) mixed condensates containing polyether and polyester blocks.
These compounds are reacted with an excess of dior polyisocyanate, optionally in a solvent which is unreactive with respect to isocyanate groups. Representative such di-- or polyisocyanates are:
(a) 1,3- or 1,4-diisocyanatobenzene;
(b) 2,4- or 2,6- or 2,5-diisocyanatotoluene (or alkylbenzene); and
(c) 4,4'-diisocyanatodiphenylmethane, whether in pure state or containing the 2,4' and 2,2' isomers.
In the condensates under consideration, tolylene diisocyanate is most frequently employed, either in the form of the pure 2,4 isomer, or as an 80/20 or 65/35 mixture with the 2,6 isomer (TDI). Mixtures of di- or polyisocyanate can also be used.
The reaction of the excess polyisocyanate with the antagonist compound is carried out according to known means, namely, by heating admixture of both starting materials, optionally in the presence of a catalyst and/or of a solvent. Another type of condensate is that referred to as biuret and which has the following structural formula (I): ##STR1## in which R is the residue of an aromatic diisocyanate.
Such condensates are produced by the reaction of water with an excess of diisocyanate. The water may be used as such, with or without a solvent. The water can also be replaced with other biuretization agents such as, in particular, tertiary alcohols, amines, formic acid and hydrogen sulfide.
Another representative type of condensate can be produced by partial cyclotrimerization of a diisocyanate under the influence of basic or organometallic catalysts, in a solvent medium where appropriate. An isocyanurate having the following structural formula (II) is thus produced: ##STR2## wherein R is as defined above. The catalysts which may be employed are described, for example, in Russian Chem. Rev., 41, (9), pages 776 and 777 (1972), or in Newer Methods of Preparative Organic Chemistry. vol. VI, pages 280 to 284 (1971).
The common feature of all of these condensates containing free NCO groups is that, when the reaction is complete, they contain a more or less considerable amount of the diisocyanate employed in excess. This diisocyanate must often be removed from the resulting polycondensate, especially because of the toxicity due to the volatility of such diisocyanate. In fact, when such precondensates produced from a diisocyanate such as TDI are used, the presence of a more or less considerable amount of TDI can present substantial hazards. These hazards are very great when the precondensate is used as a thin layer, as is the case with paints and varnishes, for example, because the emission of toxic TDI vapors can become considerable and can exceed the legally permitted limits in the surrounding atmosphere.
Another undesirable situation can arise from the use of such precondensates, especially those produced by reaction of polyesters or polyethers with an excess of diisocyanate (these precondensates being most often designated prepolymers), for the manufacture of elastomeric materials, cellular or otherwise. In this case, the prepolymer is reacted with a stoichiometric amount of a reactant which is at least difunctional and which contains functional groups reactive with NCO groups, as indicated above. The final material will contain a more or less appreciable amount of the product of condensation of the excess diisocyanate with the difunctional reactant, the presence of which may adversely affect the properties of the final elastomeric material. Thus, removal of the free diisocyanate will be the only way to obtain a prepolymer having a low degree of condensation and a low free diisocyanate content at the same time.
The removal of excess diisocyanate may be carried out by known means such as evaporation, or extraction with a solvent for the diisocyanate but a nonsolvent for the condensate. However, evaporation requires the use of high temperatures, which may impair the quality of the precondensate. This disadvantage is all the more evident when the isocyanate employed is aromatic, because aromatic NCO groups are highly reactive and result, via secondary reactions when heated, in a highly viscous, or even resinous, product which can block the evaporator. Moreover, good exhaustion of the free diisocyanate requires the use of moving thin-film evaporators; these are costly devices and, in this case, require frequent stoppages and cleaning operations, because most typically the molten condensate gradually forms a resin in the apparatus. Furthermore, extraction with a solvent for the diisocyanate but which is a nonsolvent for the condensate (such as hexane, octane, etc.) is lengthy and cumbersome. In fact, as soon as the nonsolvent is added, the condensate tends to precipitate in the form of a sticky mass from which the free diisocyanate monomer is difficult to extract completely.